nothing happens when I try to undo or correct a brush stroke on a layer mask

When I want to lighten something such as eyes or teeth in a photo, I create a new layer, make it a screen layer to lighten everything, then add a black layer mask and using the white setting paint the eyes and teeth to whiten them. For some reason when I switch back to black to undo or correct a stroke it doesn't remove the stroke...can anyone help?

I ﻿never﻿ remember what I was doing or why I changed something like that <BG>, but I've learned over time to check on it. Things go wrong, I try to remember to make my eyes circle the monitor—tools (I've had the eraser or smudge tool selected and wondered why I couldn't paint), Options bar, and back over to my settings in Layers. Usually the problem is somewhere in that circuit.